1. Field of the Invention
The present invention relates to DC-to-DC power converter circuits for regulating central processing unit (CPU) core voltages, and more particularly, to the determination of load current and power consumption by a CPU core without use of a sense resistor.
2. Description of Related Art
Switched mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode converter provides a regulated DC output voltage by selectively storing energy by switching the flow of current into an output inductor coupled to a load. A synchronous buck converter is a particular type of switched mode converter that uses two power switches, such as MOSFET transistors, to control the flow of current in the output inductor. A high-side switch selectively couples the inductor to a positive power supply voltage while a low-side switch selectively couples the inductor to ground. A pulse width modulation (PWM) control circuit is used to control the gating of the high-side and low-side switches. Synchronous buck converters generally offer high efficiency and high power density, particularly when MOSFET devices are used due to their relatively low on-resistance.
Synchronous buck converters are particularly advantageous for use in providing power to electronic systems, such as microprocessors having demanding power requirements. Conventional microprocessors, or core CPU's, generally require a voltage (VCC) of 1 to 1.5 volts with current ranging from 40 to 60 amps and can have highly transient power demands. When the core CPU executes certain operations, current demand can ramp up as much as 50 amps at a slew rate of approximately 1 amp per nanosecond. The current demand can also ramp down just as quickly after the microprocessor operations are completed. This rapid change in current draw has a direct effect upon the output voltage. Specifically, when there is a rapid demand for current, this pulls the output voltage downward, a phenomenon known as “voltage droop.” Conversely, when the current demand is curtailed, the output voltage swings upward. Conventional core voltage regulators make use of active voltage positioning, a method in which the regulated output voltage decreases linearly with an increase in load current. The load-line defines voltage input (VCC) as a function of current draw (ICC) for an integrated circuit product such as a microprocessor. The ratio of change in voltage with respect to the change in load current is referred to as the load-line slope.
In some applications, it is necessary to provide real-time measurement of CPU current as well as power consumption. There are several known ways to measure the output load current. One such approach is to include a sensing resistor in series with the output inductor and to monitor the voltage drop across the sensing resistor. The sensing resistor must have a resistance value large enough to keep the sensed voltage signal above the noise floor, as the voltage drop can be measured more accurately with a higher resistance value. A significant drawback of this approach is that the sensing resistor wastes the output energy and thereby reduces the efficiency of the synchronous buck converter. Moreover, the sensing resistor generates heat that must be removed from the system.
Another approach to measuring the load current is to place the sensing resistor on the input side of the converter in series with the drain of the high-side switch (i.e., MOSFET) and monitor the voltage drop across the sensing resistor as in the preceding approach. In this position, the amount of energy dissipated by the sensing resistor is substantially less than in the aforementioned position in series with the output inductor. A drawback of this approach is that the high-side switch changes state at a relatively high rate (e.g., greater than 250 KHz) and, as a result, the high-side switch current is discontinuous. When the high-side switch turns on, the current through the switch and the sensing resistor starts at zero and increases rapidly before settling and then returning to zero when the high-side switch turns off. The information obtained from sampling the voltage across the sensing resistor must therefore be utilized during a subsequent switching cycle, making it necessary to include “sample and hold” circuitry to store the sampled information from cycle to cycle. Not only does this add complexity to the converter, but there is also a time delay in regulating the output current that diminishes the stability of the converter. Additionally, this approach becomes difficult to implement and hence impractical at very high switching rates (e.g., approximately 1 GHz) due to bandwidth requirements.
It is also known to use the internal resistance (RDSON) of the MOSFET switches as a sensing resistor. The advantage of this method is that there is no additional loss in energy by using the RDSON as the sensing resistor since this energy loss is already an inherent part of converter operation. Due to the low duty cycle of the MOSFET switches, it is generally necessary to use the low-side switch as the sensing resistor. The voltage drop across the low-side switch is measured and averaged using a slow time loop to sense the output current. While this approach provides an accurate measurement of output current, it is generally too slow to provide effective information for current load control.
Accordingly, it would be desirable to provide real-time output current and power measurement for a CPU core powered by a DC-to-DC power converter having active voltage positioning without these drawbacks of the prior art.